re-add missing usings

Author: adamsitnik

src/libraries/Microsoft.Extensions.Configuration.Json/tests/JsonConfigurationTest.cs

@@ -4,8 +4,10 @@
 using System;
 using System.Globalization;
 using System.IO;
+using System.Linq;
 using Microsoft.Extensions.Configuration.Json;
 using Microsoft.Extensions.Configuration.Test;
+using Microsoft.Extensions.FileProviders;
 using Xunit;
 
 namespace Microsoft.Extensions.Configuration